A study of the evolution of phages of Corynebacterium diphtheriae using DNA characterization techniques (hybridization, heteroduplexing and restriction endonuclease digest patterns) will be extended to include a diverse series of phages. These have previously been characterized as differing in such characteristics as morphology, serology, phage immunity and the ability to convert non-toxinogenic C. diptheriae to toxinogeny. These studies not only bear on the evolution of corynephages but on the mechanism by which the gene for diptheria toxin has been transmitted from phage to phage and possibly from host to phage. A second study, concerning a group of phage mutants now sensitive to repression by a previously heteroimmune phage, will also continue. This study bears closely on the sequence of stages by which phages may evolve, and more specifically on the evolution of the corynephages. Efforts to develop a system for the genetic analysis of C. diptheriae are continuing, and are ultimately crucial to the analysis of the host contribution to the regulation of toxin synthesis, particularly to the role of metal ions in regulation.